Conditions for the importance of inelastic losses in photoemission

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It is shown that for materials near the metal insulator transition, inelastic losses can give rise to substantial changes on a scale of 10-100 meV in the observed lineshape compared to the intrinsic spectral function of an electron photoemitted from a solid. These changes arise from the frequency dependence of the loss function, similar to those directly observed in electron energy loss experiments. For good metals and insulators, on the other hand, the inelastic losses not important. We derive quantitative conditions to delineate the various cases. We find in particular that La₁.₂Sr₁.₈Mn₂O₇ does not satisfy these conditions, so that no significant losses are to be expected in this material. This contradicts the reasoning of Schulte et.al [Phys. Rev. B 63, 165429 (2001)], who, on finding no significant lineshape effects in this material, concluded that inelastic losses are always unimportant.